Tumor is a kind of major diseases which threaten human health, and tumor treatment has been attracting close attention all around the world. Traditional chemotherapeutic drugs block cell division non-specifically and thereby cause cell death, which damages the normal cells of the human body while killing the tumor cells. In addition, quite a few cytotoxic drugs have a limited therapeutic scope, and can easily cause adverse reactions. Long-term administration of such drugs will also result in drug resistance.
In recent years, pathogenesis of tumors is further understood in the cellular and molecular level with the rapid development of combinatorial chemistry, molecular biotechnology, structure-based drug design, computer science and other technology, so that biotherapy for tumor has made great progress and reached a new era of molecular targeting therapy. Targeted anticancer drugs can target specific pathways, prevent tumor growth and reduce toxicity to normal cells. They have common characteristics which are: non-cytotoxic and tumor cell-targeting; having effect on cell regulation and stabilization; generally can not be achieved dose-limiting toxicity and maximum tolerated dose in clinical studies; able to kill tumor cells which are not sensitive to or having resistance to chemotherapy, and resulting in better efficacy in combination with the conventional treatments (radiotherapy, chemotherapy).
At present, many anti-tumor targeting sites have been found, and histone deacetylase and microtubule are important targets for developing new anti-cancer drugs.
Factors that cause tumor gene expression and abnormal activity of gene expression products are two major changes, i.e., genetic and epigenetic changes. Among them, epigenetics refers to a way of gene expression regulation, which affects transcriptional activity of a gene without involving DNA sequence changes. The molecular basis is mainly related to two aspects: one is DNA methylation modification, and the other is acetylation modification of chromatin histones. Acetylation and deacetylation of chromatin histones is one of the key processes in regulating gene expression, in which two enzymes, i.e., histone acetyltransferase (HAT) and histone deacetylase (HDAC), determine the degree of histone acetylation. Histone deacetylase is a kind of enzyme catalyzing removal of an acetyl group from lysine on histones. It plays a critical role in chromatin condensation and chromatin remodeling as well as in the gene regulation involved, and is an important part of epigenetic regulation. Abnormality of this regulation mechanism is closely related to tumorigenesis and tumor development. HDAC includes four classes and 18 different isoforms (Class I: HDAC1, 2, 3, 8; Class II: HDAC4, 5, 6, 7, 9, 10; Class III: Sirt1-7; Class IV: HDAC11). HDAC and histone acetyltransferase (HAT) regulate histone acetylation modification together, wherein, HAT acetylates specific lysine residues on histones, while HDAC is responsible for removal of such modification for the residues (J Mol Biol, 17-31, 2004, 388:1). Histone acetylation leads to loosening of the chromatin structure and thereby facilitates binding of other DNA-binding proteins, which has a deacetylation effect on multiple proteins in the cytoplasm, such as tumor suppressive factor p53, molecular chaperone proteins (Johnstone & Licht, Cancer Cell 4, 13-18, 2003), DNA damage repair protein Ku70 (Kerr et al., Cell Death and Differentiation 19, 1317-1327, 2012), microtubule protein a-tubulin. In tumors, coincidentally, the deacetylation of various proteins in the cytoplasm usually results in an impact of facilitating tumor resistance to chemotherapeutic drugs or escaping from programmed cell death, for example, deacetylation of p53 protein will promote the degradation of the protein (Kim et. al., Apoptosis 18, 110-120, 2013). Thus, small molecule drug development focusing on such important epigenetic inheritance affecting molecular targets such as HDAC, has becoming a hotspot nowadays in the field of tumor targeting therapy worldwide.
Histone deacetylase inhibitor is one of the hotspots in the field of anti-tumor drug research in recent years. Studies have shown that histone deacetylation inhibitor can effectively inhibit tumor cell proliferation, induce tumor cell differentiation and apoptosis and anti-tumor angiogenesis, and has inhibitory effect on migration, invasion and metastasis of tumor cells. It can be divided into four categories: (i) hydroxamic acid analogues, wherein the representative compounds include SAHA (licensed in 2006 for CTCL), Panobinostat (licensed in 2015 for treating multiple myeloma), Belinostat (Phase II clinical trials), and etc; (ii) benz amide analogues, wherein the representative compounds include Entinostat (Phase II clinical trials), Mocetinostat (Phase II clinical trials), Chidamide (licensed in 2014 for treating CTCL), and etc; (iii) cyclic peptides, wherein the representative compound includes Romidepsin (licensed in 2009 for CTCL); (iv) aliphatic carboxylic acids, wherein the representative compounds include Valproic acid (Phase III clinical trials), VP-101 (Phase II clinical trials), and etc. In addition, there are some inhibitors not embraced by the above-mentioned four categories, such as RG2833 (Phase I clinical trials), CXD101 (Phase II clinical trials) and etc., due to their particular molecular structures. (Giannini et al, Future Med. Chem. 4 (11), 1439-1460, 2012; Zhiming Li et al, Int. J. Biol. Sci. 10 (7), 757-770, 2014).
SAHA (also known as Vorinostat) falls into the hydroxamic acid category, and is the first histone deacetylase inhibitor licensed for treating cutaneous T cell lymphoma (CTCL), its application in treating solid tumors isalso in clinical trials. Chidamide was licensed in December 2014 for treating CTCL. In February 2015, Panobinostat was licensed for treating multiple myeloma. Romidepsin (FK228) developed by Celgene (USA) is a histone deacetylase inhibitor of the cyclic tetracycline category, and was licensed for treating CTCL in USA in 2009 and licensed by the US FDA in 2011 for treating recurrent/refractory peripheral T cells lymphoma (PTCL). Since SAHA, Panobinostat and FK228 are all non-selective inhibitors for HDAC and can inhibit a number of signaling pathways, they have strong toxic side effects. Clinical data have shown that SAHA can cause thrombosis and neurotoxicity, while for FK228, the incidence of medication-related adverse reactions above Phase III is up to 66% and it has cardiotoxicity. In addition, for both of the drugs SAHA and FK228, their absorption peak concentrations, which directly associated with clinical validity, are significantly higher than the concentrations required for inhibiting the growth of normal or tumor cells in vitro, and thereby result in direct cytotoxicity to normal cells. This increases toxic side effects of drug administration and severely limits their use in a comprehensive tumor treatment in combination with other drugs having different mechanisms. The pre-clinical animal trials of Entinostat (MS-275, a benzamide-based HDAC inhibitor developed by Bayer (Germany) and Syndax, USA) show that the compound has significant anticancer activity against hematopoietic cancer, lung cancer and rectal cancer, and is a selective HADC inhibitor. Since its elimination half-life in human body is approximately up to 100 hours and it has a huge difference in drug exposures, it exhibits poor tolerance in clinical trials in human and therefore the administration dose cannot be increased. Chidamide, a novel anti-cancer drug innovated in China, is the first licensed isoform selective histone deacetylase inhibitor for oral administration in the world, and also the first Chinese innovator drug whose patent right was authorized to USA and other developed countries, it is available in the market in December 2014. This marks that the core technologies and capability of integrating the whole process of structure-based molecular design, targeting site research, safety evaluation, clinical development, and industrialization in our country are significantly improved, and is a historic breakthrough in the pharmaceutical industry in China. So far, there are a variety of HDAC inhibitors available in the market or undergoing clinical evaluation.
The HDAC inhibitors of the hydroxamic acid category consist of enzyme surface recognition region (Cap), a linking region (Linker) and zinc ion binding region (ZBG). In order to find the compounds having better activity than SAHA inhibitor, the researchers have done considerable studies on such compounds, wherein the main research of the hydroxamic acid inhibitor is focused on structural optimization of the enzyme surface recognition region and the linking region while keeping the hydroxamic acid groups in the metal binding region unchanged, in order to discover the derivatives with stronger activity and higher selectivity and safety. How to improve anti-tumor activity as well as reduce the impact on normal tissues or cells is an issue of great concern.